Parse a model selection expression. Model selection expressions are of the form:
            "metric cmp threshold -> model; metric cmp threshold -> model; model;"
        That is, any number of conditional expression selections followed by up to one unconditional
        selection expression, separated by semicolons. Expressions are processed from left to right.

        - Supported metrics are "pixels" - the number of pixels in the larger dimension of the table, and
        "chars" - the number of characters in the table, as detected by the partitioner's text_extractor.
        - Supported comparisons are the usual set - <, >, <=, >=, ==, !=.
        - The threshold must be numeric (and int or a float)
        - The model must be either "deformable_detr" or "table_transformer"

        Args:
            selection: the selection string.

        Returns:
            a function that can be used to select a model given the pixels and chars metrics.